A fuel cell refers to a device that electrochemically induces oxidation reaction of a fuel and converts free energy generated in the oxidation reaction to electric energy. The fuel cell has been developed into various types and structures, including a phosphate fuel cell, a polymer electrolyte fuel cell, a molten carbonate fuel cell and a solid oxide fuel cell.
The solid oxide fuel cell is operated at an elevated temperature of about 600 to 1,000° C. to produce electric energy and thermal energy. Among the fuel cells developed thus far, the solid oxide fuel cell exhibits the greatest energy conversion efficiency. Owing to the increased energy conversion efficiency, the solid oxide fuel cell will be able to replace the existing energy conversion devices if it is put into practical use. Furthermore, if hydrogen is used as fuel in the solid oxide fuel cell, it becomes possible to reduce emission of carbon dioxide (CO2). Thus, it is expected that the solid oxide fuel cell will be used as an energy source in the future energy systems.
In the meantime, the solid oxide fuel cell has an advantage in that it can use natural gas, coal gas or the like as well as hydrogen. This is because the solid oxide fuel cell is operated at an elevated temperature and, therefore, reaction can occur within a fuel electrode (i.e., an anode). Unlike the molten carbonate fuel cell, the solid oxide fuel cell does not use liquid electrolyte and has an advantage in that it is free from the problems of corrosion of materials, loss of electrolyte and replenishment of electrolyte. High quality waste heat dissipated from the solid oxide fuel cell can be recovered and used in combined power generation, thereby increasing the efficiency of an overall power generation system.
In principle, the solid oxide fuel cell is comprised of individual electric cells each including an oxygen ion conducting electrolyte layer, an air electrode (i.e., a cathode) and a fuel electrode, the latter two of which are arranged on the opposite surfaces of the former. If an air and a hydrogen fuel are supplied to the electrodes of the individual electric cells, reduction reaction of oxygen occurs in the air electrode to thereby generate oxygen ions. The oxygen ions are moved to the fuel electrode through the electrolyte layer and then reacted with hydrogen supplied to the fuel electrode, consequently producing water. At this time, electrons are generated in the fuel electrode and consumed in the air electrode. Thus, an electric current can be generated by interconnecting the fuel electrode and the air electrode.
In such a solid oxide fuel cell, the electrolyte layer is required to have ion conductivity great enough to permit passage of the oxygen ions therethrough. In order to reduce resistance against passage of the oxygen ions, it is also requested that the electrolyte layer be formed into a thin film having the smallest possible thickness within a range of assuring mechanical durability.
Depending on the type of a fuel cell stack, solid oxide fuel cells are largely divided into two kinds, i.e., tubular solid oxide fuel cells and planar solid oxide fuel cells. The tubular solid oxide fuel cells can be found in a large number of patent documents, including, KR10-0286779B and KR10-0344936B. The planar solid oxide fuel cells are disclosed in, e.g., KR2000-0059837A.
The planar solid oxide fuel cells may be classified into a self-standing type fuel cell and a support body type fuel cell. Individual electric cells of the self-standing type fuel cell are manufactured by coating an anode and a cathode each having a thickness of several tens micrometers on the opposite sides of an electrolyte substrate having a thickness of at least about 200 μm. Individual electric cells of the support body type fuel cell are manufactured by forming a thin electrolyte film having a thickness of about 20 μm on a porous electrode support body having a thickness of 1 to 2 mm.
However, the conventional solid oxide fuel cells including the planar ones disclosed in the above-noted patent documents suffer from a number of knotty problems in that it is difficult to reduce the thickness of the electrolyte layer and to enlarge the electrode area in an effort to assure high efficiency power generation. For example, in case the surfaces of the electrodes are coated with electrolyte and heat-treated to reduce the thickness of the electrolyte layer, an increased number of internal defects arise from the difference in thermal behaviors between the electrode material and the ion conducting material. Particularly, the electrolyte coating and heat-treatment entails a drawback that small pinholes are generated in the electrolyte layer formed on the electrode material and another drawback that the fuel makes direct contact with an air through the pinholes, consequently reducing the energy conversion efficiency. In case of co-firing the electrodes and the electrolyte layer, there is a problem in that the properties of the electrolyte layer are changed due to the reaction between the electrodes and the electrolyte layer, which leads to reduction in the ion conductivity of the electrolyte layer.
In order to increase the output power of a solid oxide fuel cell, there is a need to interconnect a plurality of individual electric cells in such a way that the resultant fuel cell stack can have a broad area. Although metal is used in connecting the individual electric cells, this poses a problem in that durability of the solid oxide fuel cell is deteriorated due to the difference in thermal expansion coefficient between the electrode constituting material, i.e., ceramic, and the cell connecting metal. In particular, a problem is posed in that the solid oxide fuel cell may possibly be broken by the thermal stress developed when the fuel cell is repeatedly activated and deactivated.